In pregnancy, for diagnostic purposes, fetal cells can be obtained by methods such as amniocentesis or chorionic villus sampling. With such methods, it is relatively easy to obtain a significant sample for diagnostic analysis, so that diagnosis is highly reliable. But, because they require invasion of the uterus, these methods pose a risk especially to the fetus.
Fetal cells are present also in the maternal circulation. Thus, in principle, a maternal blood sample may be used for fetal diagnosis, obtained by venipuncture. However, because of the low concentration of fetal cells, it is difficult to extract a significant sample of fetal cells from maternal blood.